1. Field of the Invention
This invention relates to circuit boards for electrical instrument control, and more particularly to such circuit board which has a pattern structure to apply a high voltage thereon.
2. Description of the Prior Art
As the circuit boards for electrical instrument control are often included with the pattern structure to apply the high voltage thereon, this has produced a problem of faulty operations of the other circuits and damages due to leakage. According to the prior known proposals for improvements, the high voltage patterns have been made up in the form of shield wires, and an additional circuit for protection has been incorporated into the board. But such methods have disadvantages in cost and the board area. It has, therefore, been desired to improve them.
In FIG. 1, as an example of the conventional circuit inclucing the pattern structure on which is applied the high voltage there is schematically shown the circuitry of the popular flash unit for camera. In the art of cameras it has been the common practice that the shutter device is provided with a switch 1 for producing an actuating signal for firing the flash unit when the leading curtain of the shutter has run down, or so-called X-synchro switch. When this X-synchro switch 1 is turned on, the trigger circuit in the flash unit produces a firing start signal, thereby the flash tube is caused to emit flash light. The flash unit 100 of FIG. 1 is constructed with a trigger circut 110 of a trigger condenser 112 and a transformer 114, a discharge tube 120, a main condenser 130, a boosting circuit 140, an electrical power source 150 and an adjusting resistor 160. By turning on the X-synchro switch 1, the trigger circuit 110 is actuated, and the boosted voltage by the booster circuit 140 is given through the main condenser 130 to the discharge tube 120 to emit flash light. It will be here understood that at the X-synchro switch 1 there appears the high voltage (usually 300 to 500 volts) for application across the ends of the discharge tube 120. Since the operation of the X-synchro switch 1 is controlled by the shutter control device constituting part of the interior of the camera the high voltage is applied to that pattern structure of the circuit board of the shutter control device which associates with the X-synchro switch 1.
Recently cameras are highly electronized with the use of CPU so that the complicated camera mechanisms are controlled based on the various information inputs from the various interfaces. In the shutter mechanisms, too, the main stream is to the electronically controlled shutter of which the leading and trailing curtains are driven to move by electromagnetic powers. The electrical signals for controlling these mechanisms take the form of very minute current between the interfaces as the electronization advances to high degree. In such electronized camera, however, the use of a circuit arrangement of permitting the high voltage as from the above-described flash unit to be applied to the patterns of the circuit board within the camera provides a source of producing various problems. For example, in the electromagnetic drive type of shutter, when the flash unit turns on in synchronism with the leading curtain running signal to apply the high voltage to the X-synchro switch, the voltage level of the trailing curtain running signal line is caused to vary. As a faulty operation takes place, the trailing curtain runs down. This faulty operation leads to the failure of obtaining the full open aperture of the shutter necessary to make a flash exposure, and, therefore, to a problem that the film frame cannot be exposed uniformly over the entire area thereof. Also, the aforesaid high voltage of the X-synchro switch changes the ground level of the CPU through the ground line from the X-synchro switch, giving a bad influence to the other camera operations, and, in the case of the CPU having RAM, causing the memory to forget.
To improve these drawbacks, according to the prior known proposals, use is made of shield wires between the X-synchro switch and the X-contact of the camera. Another counter-measure is taken that a filter circuit for protection is incorporated into the drive circuit of the shutter. These proposals have produced a new problem of increasing the cost and the area of the circuit board. Also, even when the above-described improvements have been done, it is avoided from the point of view of safety that, in actual practice, the X-synchro switch is put into the circuit board such as that for the shutter drive circuit. As it is separately constructed in another circuit board, when adjusting or checking the shutter unit, the operation has to be carried out by using the above-described shield wire. This results in increasing the number of steps in the assembling process, which in turn builds a large barrier to the progress of automation of assembling and adjusting the shutters. It has, therefore, been strongly desired to improve this.